I have a 1988 Nissan 200sx with 140,000mi. Up until yesterday it ran great, but today I noticed fluctuations in the oil pressure gauge. At idle, and at low rpm it registers ~30 - 45psi…quite reasonable for a car of its age and mileage. However, on the highway, the pressure drops to near 0. I had the oil filter/oil changed and there were no obvious leaks. The pump may be getting tired…but still functions well at low rpms. This leads me to believe that the high pressure release valve may be gunked/stuck open. I have consulted a few on-line mechanic help type site and they seem to concurr.
Does anyone out there have any experience with there types of problems? Are there any good oil additives that may de-gunk the problem (if, of course that is where the problem lies.)
I would offer that a little more pursuit before adding any additives to your oil would be the best option.
Oil pressure sending units can behave in most illogical ways sometimes. A very small leak anywhere between the engine block and the diaphragm that moves in response to pressure change, could allow for the low versus high rpm differences. A tiny rupture in the diaphragm could hold pressure at low rpm but let the pressure equalize on both sides at higher rpm, with no external leaks visible. Your engine is old enough to possibly be dirty to the degree that a new leak might not be readily detectable unless you are looking hard. The lost quantities of oil might not even be noticeable as compared to the overall consumption in that engine.
My guess is that a new sending unit might cost between 30 and 80 dollars (US). If you have any friend with a similar vintage(plus or minus several years) Nissan, borrowing the good unit for a test would be an option. Someone with a manual gauge on a short hose with the appropriate threads could test the pressure very easily and cheaply. If the concept of sending units, and uninstalls/reinstalls, and service manuals is not your cup of tea, seek outside help.
The probability of a pump problem is low on the totem pole if the car got reasonably regular oil and filter changes. Nonetheless, you must know that the damage to your engine could be fatal to your wallet if you drive it anymore without resolving at least the question.
I agree with Bawdy - look to the sending unit first. This behavior sounds like what I have seen before with bad sending units. Oil pumps themselves typically do not wear out or fail or plug. And when they fail, it is typically suddenly and completely.
I would personally never do what Tedster suggests here. I think your engine might end up leaking and/or burning oil like you would not believe afterwards.
You didn’t mention,Does the car smoke? Is there a loss of power?
Both would be indicative of no oil pressure.
I agree with most here about the oil sending unit. As far as the kerosene, that is an old trick to free sticky valves. You can get the same results with top oil usually available at auto parts stores,wal mart etc.
I would suspect a bad sending unit. Because if you really had no oil pressure at high RPM, you would know it, believe me.
The first thing you ought to do is stick a mechanical oil pressure gage (preferably calibrated) on your engine. This will let you know what’s really going on with the oil pressure.
A word about additives…
I have heard from two very reputable sources (one of whom is a master mechanic and hosts a weekly radio show) that the best engine de-sludger is automatic transmission fluid (ATF). That’s right: fill your crankcase with ATF, run it for 50 miles, and drain. According to these guys, you would be amazed at how well ATF cleans up your system. And that’s why you don’t want to run it more than 50 miles; ATF is so rich in detergents, and so effective at de-sludging your engine, that your oil filter will get clogged very quickly.
Well, there is no indication of Total Loss of oil pressure. It does not smoke, there is no loss of power. There is no knocking or other engine noise. It does not stall or overheat. I’m hoping it’s the sending unit…I think I could afford to replace one of those.
S.
I should know by now not to second-guess Anthracite, but I’m hoping that he’s slightly off his game (it is Monday, afterall).
I’ve seen this behaviour in motors where “gunk” has built up in the head(s) and has partially clogged the oil return passages. The oil doesn’t drain back to the pan rapidly enough to keep the sump filled at higher RPMs. This can cause pretty extensive damage to bearing surfaces pretty quickly, so should not be ignored.
With the car parked, run the RPMs up and look into the valve cover to see if oil is accumulating there.
I’ve used the ATF trick successfully, although never at more than 1qt ATF/3qt oil. (And here I agree with Anthracite, you may have new leaks afterwards.)
If that doesn’t work, It should be pretty easy to remove the valve cover and clean the stuff out manually.
As far as additives go, I don’t trust any of them. The ATF idea sounds reasonable since it is basically 30W oil with some detergents and antifoaming stuff in it but as far as buying some crap to pour into your oil and “rebuild” your engine, don’t do it.
I rebuilt the engine in my 1990 Ford Ranger after the oil change dude at Express Oil Change talked my wife into this “powerful” new oil additive. Yeah, it was powerful alright, it made the cork oilpan gasket deteriorate in about a week and the cork fell into the oilpan. Guess where it went after that?
That would be an engine that would be catastrophically clogged up - something I have very rarely seen. And most of the time (IME) it is very hard to see anything meaningful via the valve cover oil fill hole.
I guess I feel better about using ATF than a kerosene mixture.
I will 3rd (or is it 4th) the sending unit. As a longtime Nissan owner, I will do better than guess though… I can almost guarantee it. I would be extremely suprised (bordering on shock) if a Nissan motor with 140K had a oil pump or valve seal faliure.
Go to a local parts shop (not necessarily Nissan) and buy one. Tell them you have the gauge instead of the idiot light. It should cost you about $25 or so.
